Can't Calculate A Negative Value

I have 3 textbox controls on my web page that are numeric currency fields in which a user can enter a numeric value, either positive or negative into TB1 and TB2 (both of these fields go thru edit checks to make sure a numeric value is entered and then return a currency value ie; $2500.00). TB3 is calculated resulting in TB3 = TB1 (($6500)) - TB2 ($2500).

My problem is that if a user enters a negative value for TB1, it returns a "NaN" and the value in TB3 turns out to be the same value as in TB2. Essentialy the calulation ends up being 0 - 2500 = 2500 (see screenshot). I've attached 2 functions below.

Code:
function cal_Overage_Shortage() {
if (make2Number("TB_LowPoint") <= 0) {
document.getElementById("TB_OverageShortage").value = GetValueTextbox("TB_LowPoint") + GetValueTextbox("TB_RequestedLowPoint");

[Code]....